[PATCH 3.2 077/104] module/retpoline: Warn about missing retpoline in module

[Ben Hutchings]

3.2.101-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

commit caf7501a1b4ec964190f31f9c3f163de252273b8 upstream.

There's a risk that a kernel which has full retpoline mitigations becomes
vulnerable when a module gets loaded that hasn't been compiled with the
right compiler or the right option.

To enable detection of that mismatch at module load time, add a module info
string "retpoline" at build time when the module was compiled with
retpoline support. This only covers compiled C source, but assembler source
or prebuilt object files are not checked.

If a retpoline enabled kernel detects a non retpoline protected module at
load time, print a warning and report it in the sysfs vulnerability file.

[ tglx: Massaged changelog ]

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: gregkh@xxxxxxxxxxxxxxxxxxx
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: jeyu@xxxxxxxxxx
Cc: arjan@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20180125235028.31211-1-andi@xxxxxxxxxxxxxx
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/cpu/bugs.c | 17 ++++++++++++++++-
 include/linux/module.h     |  9 +++++++++
 kernel/module.c            | 11 +++++++++++
 scripts/mod/modpost.c      |  9 +++++++++
 4 files changed, 45 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -10,6 +10,7 @@
 #include <linux/init.h>
 #include <linux/utsname.h>
 #include <linux/cpu.h>
+#include <linux/module.h>
 
 #include <asm/nospec-branch.h>
 #include <asm/cmdline.h>
@@ -235,6 +236,19 @@ static const char *spectre_v2_strings[]
 #define pr_fmt(fmt)     "Spectre V2 mitigation: " fmt
 
 static enum spectre_v2_mitigation spectre_v2_enabled = SPECTRE_V2_NONE;
+static bool spectre_v2_bad_module;
+
+#ifdef RETPOLINE
+bool retpoline_module_ok(bool has_retpoline)
+{
+	if (spectre_v2_enabled == SPECTRE_V2_NONE || has_retpoline)
+		return true;
+
+	pr_err("System may be vunerable to spectre v2\n");
+	spectre_v2_bad_module = true;
+	return false;
+}
+#endif
 
 static void __init spec2_print_if_insecure(const char *reason)
 {
@@ -420,6 +434,7 @@ ssize_t cpu_show_spectre_v2(struct sysde
 	if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2))
 		return sprintf(buf, "Not affected\n");
 
-	return sprintf(buf, "%s\n", spectre_v2_strings[spectre_v2_enabled]);
+	return sprintf(buf, "%s%s\n", spectre_v2_strings[spectre_v2_enabled],
+		       spectre_v2_bad_module ? " - vulnerable module loaded" : "");
 }
 #endif
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -661,4 +661,13 @@ static inline void module_bug_finalize(c
 static inline void module_bug_cleanup(struct module *mod) {}
 #endif	/* CONFIG_GENERIC_BUG */
 
+#ifdef RETPOLINE
+extern bool retpoline_module_ok(bool has_retpoline);
+#else
+static inline bool retpoline_module_ok(bool has_retpoline)
+{
+	return true;
+}
+#endif
+
 #endif /* _LINUX_MODULE_H */
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2335,6 +2335,15 @@ static inline void kmemleak_load_module(
 }
 #endif
 
+static void check_modinfo_retpoline(struct module *mod, struct load_info *info)
+{
+	if (retpoline_module_ok(get_modinfo(info, "retpoline")))
+		return;
+
+	pr_warn("%s: loading module not compiled with retpoline compiler.\n",
+		mod->name);
+}
+
 /* Sets info->hdr and info->len. */
 static int copy_and_check(struct load_info *info,
 			  const void __user *umod, unsigned long len,
@@ -2495,6 +2504,8 @@ static int check_modinfo(struct module *
 	if (!get_modinfo(info, "intree"))
 		add_taint_module(mod, TAINT_OOT_MODULE);
 
+	check_modinfo_retpoline(mod, info);
+
 	if (get_modinfo(info, "staging")) {
 		add_taint_module(mod, TAINT_CRAP);
 		printk(KERN_WARNING "%s: module is from the staging directory,"
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -1889,6 +1889,14 @@ static void add_intree_flag(struct buffe
 		buf_printf(b, "\nMODULE_INFO(intree, \"Y\");\n");
 }
 
+/* Cannot check for assembler */
+static void add_retpoline(struct buffer *b)
+{
+	buf_printf(b, "\n#ifdef RETPOLINE\n");
+	buf_printf(b, "MODULE_INFO(retpoline, \"Y\");\n");
+	buf_printf(b, "#endif\n");
+}
+
 static void add_staging_flag(struct buffer *b, const char *name)
 {
 	static const char *staging_dir = "drivers/staging";
@@ -2210,6 +2218,7 @@ int main(int argc, char **argv)
 
 		add_header(&buf, mod);
 		add_intree_flag(&buf, !external_module);
+		add_retpoline(&buf);
 		add_staging_flag(&buf, mod->name);
 		err |= add_versions(&buf, mod);
 		add_depends(&buf, mod, modules);